1. Field
This invention relates to plasma sterilization, and provides a method for exposing articles to be sterilized to substantially neutral species of a plasma in a field free, glowless volume.
2. State of the Art
Modern medical and dental practice require the use of aseptic materials and devices, many of them meant for repeat use. In order to achieve this sterilization, processes are needed, at the manufacturer, and also at the hospitals or dental offices for treatment of reusable materials and devices.
Typical of materials which are reused in the hospital environment and require repeated sterilization are major surgical instrument trays, minor surgical kits, respiratory sets, fiber optics (endoscopes, proctoscopes, angioscopes, bronchoscopies) and breast pumps. Typical instruments and devices which are reused in a dental environment and require repeated sterilization are hand-pieces, dental mirrors, plastic tips, model impressions and fabrics.
There are a wide variety of medical devices and materials that are to be supplied from the manufacturer already packaged and sterile. Many of these devices and materials are disposable. Typical of this group are barrier packs, head coverups and gowns, gloves, sutures, syringes and catheters.
One major sterilization process in present use is that which employs ethylene oxide (EtO) gas in combination with Freon-12 (CCl.sub.2 F.sub.2) at up to three atmospheres of pressure in a special shatter-proof sterilization chamber. This process, in order to achieve effective asepsis levels, requires exposure of the materials to the gas for at least one to three hours followed by a minimum of twelve hours, or longer, aeration period. The initial gas exposure time is relatively long because the sterilization is effected by alkylation of amino groups in the proteinaceous structure of any microorganism. EtO sterilization requires the attachment of the entire EtO molecule, a polyatomic structure containing seven atoms to the protein. This is accompanied by the requirement of hydrogen atom rearrangement on the protein to enable the attachment of EtO. Because of kinetic space-hindrance factors governing the attachment of such a bulky molecule, the process needs to be carried out at high pressure and be extended over a long period of time. It is, therefore, deemed very inefficient by the industry at large.
Perhaps the chief drawback to this system, however, is its dangerous toxicity. Ethylene-oxide (EtO) is a highly toxic material dangerous to humans. It was recently declared a carcinogen as well as a mutagen. It requires a very thorough aeration process following the exposure of the medical materials to the gas in order to flush away toxic EtO residues and other toxic liquid by-products like ethylene glycol and ethylene chlorohydrin. Unfortunately, it is a characteristic of the gas and the process that EtO and its toxic by-products tend to remain on the surface of the materials being treated. Accordingly, longer and longer flush (aeration) times are required in order to lower the levels of these residues absorbed on the surface of the materials to a safe operational value. A typical volume for each batch using this EtO process is 0.2 to 50 cu. ft. within the health and dental care environments.
A number of other approaches for performing sterilization have also been employed. One such process is high pressure steam autoclaving. However, this requires high temperature and is not suitable for materials which are affected by either moisture or high temperature, e.g., corrodible and sharp-edged metals, plastic-made devices, etc., employed by the hospital and the dental communities.
Another approach utilizes either x-rays or radioactive sources. The x-ray approach is difficult and expensive. The use of radioactive sources requires expensive waste disposal procedures, as well as requiring radiation safety precautions. The radiation approach also presents problems because of radiation-induced molecular changes of some materials, which, for example, may render flexible materials brittle, e.g., catheters.
It is therefore a primary object of the present invention to provide a process and apparatus for dry sterilization of medical and dental devices and materials, which can be operated efficiently, both with respect to time and volume and which can be carried out below 70.degree. C.
It is another object of the present invention to provide a safe, nontoxic, process for the sterilization and surface treatment of medical and dental devices and materials, a process which does not employ toxic feed gases and one which does not yield toxic absorbed surface residues and by-products.